Fibrous plaster boards are board-shaped structures, which comprise a mixture of gypsum and fibers, the proportion of gypsum predominating and usually amounting to 80 to 90% and the fibers usually comprising paper fibers.
Methods are known for the production of such fibrous plaster boards, in which the gypsum is first of all calcined dry, that is, converted from the dihydrate into the beta hemihydrate. The hemihydrate is exclusively mixed either with dry or wet-refined fibers and the water required for hydration is added.
It has been ascertained that fibrous plaster boards, produced with wet digested paper fibers in the form of pulps, have 10 to 50% higher transverse bending strengths than do boards produced with fibers pulped by a dry process. Moreover, the fibers are dispersed more homogeneously in the core of the board and the appearance as well as the ease of working such panels is better. For shaping such boards, a suspension of calcined gypsum, that is, the hemihydrate, a pure paper pulp with a very high water content of about 300 to 600% is required. Such boards can be shaped only by the removal of a large proportion of the water. For this purpose, screening drums (coil method) and/or screen belts are used. With this wet method, however, it has turned out that the screen surfaces rapidly becomes contaminated by the setting gypsum, so that the expense of cleaning is appreciable and the operation is susceptible to breakdowns. To avoid these disadvantages of the wet method, methods have been proposed, for which the structures are shaped from the dihydrate.
For example, the German Offenlegungsschrift No. 2,649,300 proposes that a suspension of calcium sulfate dihydrate and fibers be shaped and dried by removal of water. This method does not utilize the hydraulic bonding properties of the gypsum, which are due to the repeated recrystallization into the hemihydrate and back into the dihydrate. For this reason, an adequate board quality cannot be achieved with this method without the addition of binders.
German Pat. No. 2,818,169 describes a method for the production of molded articles from gypsum. This method works with natural gypsum and without fibers. Only a small amount of water--5--10% of the natural gypsum--is added and high pressures of 6 Mpa are employed for the shaping. The method is intended for the production of low-pore plaster products. This method, however, is technically unsuitable for the production of fiber-containing structures of plaster and wet fibers, because wet-digested paper fibers cannot be mixed with gypsum rock in the amount of 10 to 20% required for fibrous plaster boards in such a way, that the water content in the molded article is low enough, that is, between 5 and 10%. Shaping by pressing, especially with correspondingly high pressure of 6 Mpa, is also excluded.
In the German Offenlegungsschrift No. 2,816,466, it is proposed that a board be shaped by removal of water from a suspension of dihydrate and pulp and heated under a pressure of 1.5 to 10 bar to temperatures of 115.degree. to 180.degree. C. Under these conditions, the conversion of the dihydrate to the alpha hemihydrate takes place. Subsequently, the molded article must be cooled, pressed under pressure and stored for about 2 hours to 7 days under humid room conditions and then dried. Because these storage times are too long for an industrial manufacturing process and therefore uneconomic, the German Pat. No. 3,419,558 proposes that the pH of the gypsum-fiber suspension be adjusted to a value of less than 7; after that and after the treatment of the board for 3 to 50 minutes in a saturated atmosphere, this conversion of the dihydrate to the alpha hemihydrate, that is in a pressure vessel, is said to shorten the setting time significantly. In actual fact, however, it has been ascertained that even the change in the pH has not significantly reduced the setting time; it still is about 1 to 3 days. It seems obvious to assume that, due to the wet treatment of the boards at temperatures between 100.degree. and 160.degree. C., materials are released from the paper fibers, which have a retarding effect on the setting reaction. Admittedly, due to the exchange of water in the dehydrated board, it was possible to reduce the setting time further to about 9 to 12 hours; however, this is still too long for an economic manufacturing process.